Multiplex time modulated electrical pulse demodulation system



June 13, 1950 M. M. LEVY MULTIPLEX TIME MODULATED ELECTRICAL PULSE DEMODULATION SYSTEM 2 Sheets-Sheet 1 Filed July 2, 1945 CHQICHB f I CH4|CH5 I-CHS |CH/ f f I i H CH3] f 1 CH2 f Inventor Ems LE- m F. LE1! y A Home y June 13, 1950 v M. M. LEVY 2,510,987

MULTIPLEX TIME MODULATED ELECTRICAL v PULSE DEMODULATION SYSTEM Filed July 2, 1945 2' Sheets-Sheet 2 Inventor gum iflm LEVY Patented June 13, 1950 MULTIPLEX TIME MODULATED ELECTRICAL PULSE DEMODULATION SYSTEM .Maurice Moise Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application July 2, 1945, Serial No. 602,805 In Great Britain May 26, 1944 9 Claims.

The present invention relates to a communication system of the kind in which the intelligence wave is transmitted as a time phase modulation of a predetermined change in a, characteristic of an electrical waveform in accordance with the instantaneous amplitude of said intelligence wave.

As will be now well understood by those versed in the art the predetermined change in a characteristic of the electrical waveform may comprise a change in frequency or phase such as a phase reversal, or a change in amplitude, said change being preferably sudden. In the case of a sudden change in amplitude, the system becomes the more usual and commonly known system of the kind specified, namely a time phased pulse modulation communication system.

Known methods of demodulating time phased pulses comprise adding to the received train of time phased pulses, an unmodulated train of pulses (that is umnodulated as regards their times of occurrence) of the same repetition frequency as the time phased train and then producing from pairs of pulses, each pair comprising a pulse of the modulated train and a pulse of the unmodulated train, duration modulated pulses by means of a buildback circuit. The duration modulated pulses will have one fixed edge, that is, the corresponding edge of each pulse will occur at constant intervals of time corresponding to the pulses of the unmodulated train. The duration modulated pulses are then passed through a low pass filter which gives directly in its output the desired amplitude modulated intelligence or signal wave.

An object of the present invention is to provide improved and simplified demodulating arrangements in systems of the type hereinbefore specified.

According to a broad aspect of the invention, a system of the kind hereinbefore specified has demodulating arrangements for producing duration modulated pulses under the control of the time-phased modulated wave comprising an electron discharge circuit arrangement so arranged that on the application of a high voltage thereto under the control of said predetermined change in the electrical waveform, the current through said electron discharge circuit changes from a first value to a second value and persists at said second value during the continuance of an intermediate voltage applied to said circuit arrangement, and when said intermediate voltage is removed, the current through said electron discharge circuit returns to said first value.

According to a feature of the invention a communication system of the kind in which the intelligence wave is transmitted as a time-phase modulation of a train of electrical pulses has demodulating arrangements for producing duration modulated pulses from the time-phased pulses comprising an electron discharge circuit arrangement so arranged that on the application of a high voltage thereto under the control of said time-phased pulses, a change is effected in the current in said circuit arrangement from a first value to a second value and remains at said second value during the continuance of an intermedlate voltage applied to said circuit arrangement and when said intermediate voltage is removed said current returns to said first value.

In carrying the invention into effect, the change in an electrical characteristic may be transformed in any known manner into an amplitude change, preferably a sudden amplitude change or a short sharp pulse and the derived amplitude modulation or pulse waveform may be applied to the electron discharge circuit. The intermediate voltage may comprise a train of pulses of rectangular Waveform of suitable amplitude and havin a pulse repetition frequency equal to the repetition frequency of the said chan e in electrical characteristic or corresponding derived pulses and having a duration at least as long as the time modulation limit of the time-phase modulated wave.

The electron discharge circuit arrangement may take the form of a circuit, such as a multivibrator type of circuit, of Kipp relay which is capable of being triggered from one condition to another under the control of the applied voltages.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings which show diagrammatically by way of example some embodiments of the invention.

The invention is particularly applicable to a multi-channel pulse modulation system utilising distributors at the transmitting and receiving ends in which electrical pulses are employed for defining the respective channels by defining the portion of each cyclic period which is allotted to each channel, i. e. the channel period. Such a distributor may take the form of a delay network, from points along which pulses having difierent delay times are applied to the demodulators of respective channels to bring the channels successively and cyclically into use. The

channel period is then determined by the dura tion of the pulses applied to the input of the delay network. The pulses obtained at the intermediate points along the delay network for the respective channels may be utilised as the intermediate pulses for application to the demodulation arrangements. The application of pulses to the input terminals of the delay network will, of course, be synchronised with the channel selection pulses applied to a similar delay network or other distributor at the transmitter.

In the drawings:

Figure 1 shows explanatory curves referred in the description.

Figure 2 shows diagrammatically the receiving end of a multi-channel communication system utilising time phased modulator pulses with circuit diagrams of build back arrangements er' bodying th invention respectively in channel l and channel 12.

In Figure 1 curve a illustrates a multi-channel communication system in which the intelligence waves oi the channels are transmitted as timephased modulated pulses. The system is illustrated as having six channels numbered 2' to 55 respectively. As is well known the channels are brought successively and cyclically into operation by means of a distributor at the transmitter and a distributor at the receiver working in synchronism with each other. This synchronism between the two distributors is maintained by the transmission of synchronising pulses, as illustrated at s in Figure 1, from the transmitter to the receiver. The distributors at the transmitter and receiver are preferably alike and may take the form, as described in the patent specification of co-pending British application No. 10,305/44, of a delay network or artificial line which retards the passage of an electrical current propag ted therethrough and comprises a plurality of series connected cells made up or" electrical condensers and inductances, and preferably alike, each 01" which retards the current passed therethrough by predetermined preferably equal time intervals. Such a distributor network or artificial line is indicated by numeral l in Figure 2. Pulses of distributor cyclic frequency are obtained from the master pulse generator represented by block 2 and are fed to the input terminals of the line at a constant repetition frequency and the pulses obtained at the various tapping points TPlTP6 on the network or line are then applied to bring the channels of a multi-channel system successively into use. The master pulse generator 2 may comprise, for example, a multivibrator type of circuit well known in the art, and which may be stabilised as to frequency by the pulses from the output of the network or line 5, fed back through an amplifier or reshaping device indicated by block The synchronising pulses S obtained from a synchronising pulse separator arrangement indicated by block 4 fed from the high frequency receiver and detector represented by block 5, are also applied to the master pulse generator to synchronise this latter to the transmitter distributor. The input to the receiver 5 will be obtained from whatever transmission medium is used and may be a transmission line, wave guide or radio. The output f cm the receiver is also applied in parallel to the channel equipments via respective diodes 5 which serve to isolate the respective channels from each other. Usually the amplitude of the channel pulses is limited in the detectors of recei er 5. If this is not the case, however, an amplitude limiter device indicated by block 511 is provided between the receiver 5 and the channel demodulator-s for a purpose described hereinafter. The apparatus represented by blocks i, 2, 3, l, 5 and E-a does not form part of the present invention and may take any suitable form. Hence no details are given herein.

It is convenient to employ pulses of rectangular waveform whose duration determines the channel period, that is the period of time during each distributor cyclic period when a channel is operative. Such a wave may thus be used as the pulses of intermediate voltage for the purposes of the present invention.

Such a pulse of rectangular waveform is illustrated in curbe Figure l, for channel 2, and is repeated at a repetition frequency equal to the distributor cyclic frequency. time phased pulses of the channels, 'epresented by the arrows in curve a, Figure 1, may occur between the limits t1 and t2 corresponding to the leading and trailing edges of the distributor or selector pulse. The pulse or" only channel 3 is illustrated in curve I) Figure 1 but it will be understood that the selector pulses for the other channels occur successively throughout the distributor cycle as illustrated in curves 0 and c.

Figure 2 shows in channel l a inultivibrator type of build bacl circuit comprising two interconnected valves "i and S, the anode of 7 being coupled through a capacity to the control grid of valve 8 and the anode of 8 being coupled to the control grid of valve 7. Many other versions and modifications of this circuit are well known. In its adaptation to the present invention the control grid of one of 12 valves, in the present case, the valve 8, is blessed negatively as illustrated by biassing the cathode positively with respect to earth potential by a bias battery 9 in the cathode circuit, so as normally to render the multivibrator insensitive. The same result may be obtained by a negative bias between the grid and earth potential. Ehus when the time phase modulated pulses curves a Figure l are applied to the multivibrator circuit, they are of insufii cient amplitude to render it sensitive. The selector pulses of rectangular waveform curve b Figure l, of positive sign obtained from the distributor tapping point TPi as hereinbefore explained are applied, for example, to the grid iii of valve 8 so as to reduce the negative bias, so that the multivibrator comes near enough to the threshold of sensitivity as to be trigge ed over by the first channel pulse 0?? occurring after the leading edge of the selector pulse present on the grid as illustrated in curve 0 or d Fig. i, where the triggering voltage is represented by line TV. The mutivibrator will remain in this second condition so long as the selector pulse persists, and at the end of the selector pulse t re circuit automatically triggers over to its original condition in which it remains on account of the negative grid bias of valve 3 until the next selector pulse and channel pulse occur. In the arrangement shown in Figure 2 the channel pulses are assumed to be negative and ase applied. on the grid oi valve 7. They could equally well be applied directly as positive pulses on the grid of valve 8 as in fact they are in the present illustrative case after amplification in. valve l. Channel pulses of suitable sign may of course be app ed to either of the cathode circuits as will be well understood.

In the example shown, the negative bias on the grid of valve 8 is greater than the maximum amplitude of the amplified channel pulses so that the valve 8 remains insensitive until the selector pulse occurs. The selector pulse applied on the grid of valve 8 is thus positive and has a sufiicient amplitude so that valve 8 responds to the amplified channel pulses from valve 1.

The amplitude limiter 5a renders the amplitude of the channel pulses applied to the multivibrator circuit 6 independent of the amplitude of the received pulses. Such a limiter may be, for example, an electron discharge valve arranged to have a grid swing smaller than the minimum amplitude pulses applied to its grid. The grid is negatively biassed and positive pulses are assumed applied thereto. A high resistance is inserted in series with the grid in order to limit the grid voltage swing by the grid current. If the amplified-detector of the receiver 5 has a great sensitivity the amplitude of the channel pulses applied on the grid of the limiter will always have a greater amplitude than the grid swing, and the negative pulses taken from the anode circuit of the limiter valve (5a) will have a substantially constant amplitude.

In curve 0, Figure 1, the channel pulses are represented by the vertical arrows, and the selector pulse SP2 for channel 2 is shown with the channel pulse CP2 superimposed thereon. The triggering voltage is represented by the ordinate of the broken line TV and it will be seen that the multivibrator circuit can be triggered only when the selector and channel pulses occur together. Curve d shows the similar conditions for channel 3. In curve e, Figure 1, the ordinates represent anode current and show how the anode current varies in valve 8. 2DI or 2D2 illustrates the current pulse through the anode resistance of valve 8. The voltage pulse obtained from the terminal II of the anode resistance will be negative in sign as indicated in curve 1 at DNI and DN2. The leading edge L coincides with the occurrence of the channel pulse P2 and the trailing edge T coincides with the trailing edge TI of the selector pulse SP2. Thus as successive channel pulses occur at different time intervals, and as the trailing edges TI of the selector pulses occur at constant intervals of time, the pulses of train DNI, DN2 become duration modulated, and are passed through a low pass filter represented by block I2 to produce the original intelligence wave transmitted. The output from I2 is fed to a translating device indicated as headphones I3.

Curve g, Fig. 1, shows the negative duration modulated pulses obtained for channel 2.

In channel 6, Fig. 2, is shown another multi vibrator circuit arrangement I4 and comprises a pentode valve I5 whose control grid is capacity coupled to the anode of valve I6 and the control grid of valve It is capacit coupled to the anode of valve I5. In this case the cathode of valve I5 is biassed positively by means of the resistance chain I1, is connected between H. T.+ and earth potential. Positive selector pulses are shown as applied to the control grid of valve I5, but negative selector pulses could be applied across the resistance is in the cathode circuit of valve I5. The channel modulated pulses may be applied to the control grid of valve I6. Duration modulated pulses are produced in the anode circuit of valve I5 as described in connection with Figures 1 and 2. Duration modulated pulses may also be obtained from the screen grid I9 of valve I5 as shown in the arrangement in channel 6 and may be passed through a low pass filter 20 to obtain the intelligence wave.

While the output duration modulated pulses have been described as being obtained from the anode circuits of valves 8 or I5, the pulses may equally well be obtained from the cathode cir-- cuits of these valves, depending upon whether pulses of positive or negative sign are required.-

In the case of utilising pulses of rectangular waveform as the intermediate pulses, it has beenpointed out in specification of application No. 10,305/44 that the pulses become distorted as they travel along the delay line or network. This distortion consists in an increase in the duration of the pulse, the leading and trailing edges of the waveform becoming more inclined to the time axis as the pulse proceeds along the delay network, but the duration of the pulse above substantially its mean voltage is substantially constant. A squaring or reshaping device, such as an amplifier-limiter represented by block 2I may therefore be provided. Since positive pulses have been assumed as provided by the delay network I, the output pulses of positive sign may be obtained in the cathode circuit of the amplifier limiter 2| and applied to the control grid of valve I5. Alternatively, negative pulses may be obtained from the anode circuit of amplifier limiter 2I and applied to the cathode circuit 01' valve I5.

No pulse in practice is ever obtained having a waveform with perfectly perpendicular leading and trailing edges.

If the distortion of the pulses from the delay network is not too great and the amplitude 01' the channel pulses ismaintained at substantially one half the selector pulse amplitude, interference between adjacent channels will be substantially eliminated. In curve 11 of Figure 1 a distorted selector pulse SP3 is indicated by the broken lines M in channel 3 and a channel pulse N indicated as occurring at or towards the end of channel 2. The channel duration is defined by the portion of the pulse above the mid-amplitude of the pulse SP3, and since the triggering voltage is represented by the line TV, it can be seen that if the amplitude of pulse N is made substantially half the amplitude of the selector pulse SP3, the pulse N of channel 2 can never operate the multivibrator circuit of channel 3 and interference between the channels is eliminated. similarly a pulse occurring at the beginning of channel A will be ineffective as regards channel 3.

With the arrangements utilising the selector rectangular pulse, if a pulse of a channel is missing during any channel period, the build-back circuit will not be operated and will be unaffected by the channel pulse of the next succeeding channel.

What is claimed is:

1. A system for translating time displacement modulation signal pulses into output pulses whose width varies according to said displacement comprising, means for producing constant repetition rate control pulses in synchronism with the signal pulses in their unmodulated state, each of said control pulses having a width covering the entire range of displacement of the corresponding signal pulse, a tripping circuit having two levels of stability, means for biasing said circuit to maintain it at a first one of said stability levels, said bias having a value such that the combined amplitudes of a control pulse and signal pulse are required to produce tripping to the second stability level while the control pulse alone is s-ufilcient to maintain the circuit at said second level, means for applying the control pulses and the time displacement modulated signal pulses 3. A system accore ng to claim 1, wherein said control pulses are substantially rectangular.

4. An arrangement for translating time disia1 puls s into output pulses Whose Width accorcling to said displacement, in :1 @321 ion system in which synchronizing pulses of ccnstant repetition rate are each followed by signal pulse having a time displacement wit respect to th associated synchronizing pulse that varies according to instaneous values 0- the intelligence to be conveycd cc-mpris cans for predueing under the control of a chrorizing pulses control pulses syncln'cnized said synchronizing pulses and i a is the entire range of dis sl corr sponding signal pulse, a two levels of stability, 1 said circuit to maintain it at a stability levels, said bias be such that the combined. amp itities of a control pulse and sig nal pulse are rcauired to tripping to the second level while the control pulse alone is suificient to maintain the circuit at said second placement modulation to trip s- 1C1 circ es nd level at the time of application of a signal puse and to permii; retm'n to said first level at the nd of the corresponding control ulse, and means for de riving from said circuit variable wiclth output pulses.

5. A system according to claim 4, wherein said tripping circuit a multivibrator.

6. A system according to claim l, wherein said control 3:-

placeinl pulses IOIHlTQll.

. each associated with a pin-- rality of signal pulses forming parts of separate channels and hzlVli'l" a time displacement with respect to their associated synchronizing pulse that varies accordin to the instantaneous value 5 of the intelligence to he nveyed comprising, o the control of said 0" pulses synchronized es and havin a of displacement -s, a plurality of a part of a sepatwo levels of stasaid circuits to stability levels, said bios haviLg a value such at the combined amplitudes of a control pulse and signal pulse 'ldzvt. iing to the second trol pulse alone is sufficient to at second level. means l es successively to n ep .re them we interchannel, nt modlor biasiz-i t irst 0 'ng reference patent:

UNITED STATES I 1 l. ne Date 1,5310%; am July 18, 1933 1 May '2, lQ lO De craine et a Nov. 18, 194 1 s Dec. 16, 1941 nseli .1 July 10, 1945 i. "6 Dec. 25, 1945 '5 :soii Jan. 8, 1946 6,330 Labin Feb. 25, 191'7 

